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1
Butter a 1-litre mould or four individual ramekins thoroughly, making sure the butter reaches every curve and corner so the crumb coat has something to grip. Dust the interior with fine dried breadcrumbs, then tap out any excess, turning the mould to coat evenly — gaps in the crust will cause the sformato to tear rather than release cleanly on unmoulding.
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2
Cook the spinach in a dry hot pan until fully wilted, then transfer to a colander and allow to cool. Take the spinach in both hands and squeeze with sustained pressure until no further liquid is expelled, repeating the squeezing until the spinach feels almost dry to the touch — this is where the dish lives or dies, because a sformato made from insufficiently dried spinach carries too much water to set, and no amount of care at any later stage can recover it.
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3
Chop the dried spinach finely and set aside. Melt 50 g butter in a heavy saucepan over a medium flame, add the flour, and stir constantly for 1–2 minutes to cook out the raw flour taste without letting the roux take on colour — it should smell faintly nutty and look pale. Add the milk in a slow, steady stream, whisking without pause, then continue over a gentle heat until the béchamel is thick, smooth, and pulls away from the sides of the pan.
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4
Remove the béchamel from the heat and allow it to cool briefly so that the eggs do not scramble on contact with the hot sauce. Beat in the 2 whole eggs and 2 yolks one at a time, then fold in the Parmigiano and the chopped spinach — the mixture should be uniformly pale green, thick, and cohesive, with no streaks of white or unmixed yolk.
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5
Pour the mixture into the prepared mould, filling to 80% of its height only, since the sformato rises during cooking and overfilling causes it to overflow and then collapse on unmoulding. Set the mould in a deep roasting tin and add boiling water until it reaches halfway up the sides of the mould, which keeps the surrounding heat gentle and even so the outside does not overcook before the centre has set. Place the tin in an oven preheated to 160 °C.
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6
Bake for 35–40 minutes, or 20 minutes for individual ramekins. Insert a metal skewer into the centre: it should emerge clean and dry, and the sformato should carry a slight wobble across the whole surface when gently shaken but show no liquid movement at the centre — any rippling at the centre means it needs a few minutes more.
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7
Remove the mould from the water bath and rest for 5 minutes undisturbed, so the sformato firms slightly as it cools and will release cleanly rather than slump. Run a thin knife around the inside edge, place a warmed plate over the mould, and invert in a single confident movement — lift the mould away and the sformato should stand upright and hold its shape, with a lightly crusted exterior.